Adds RTDB triggers for v2 functions

[colerogers]

Adds RTDB triggers for v2 functions.

import {
  onRefWritten,
  onRefCreated,
  onRefUpdated,
  onRefDeleted,
} from 'firebase-functions/v2/database';

export const written = onRefWritten("/foo/{bar}", (event) => {
  console.log("hi");
});

export const created = onRefCreated("/foo/{bar}", (event) => {
  console.log("hi");
});

export const updated = onRefUpdated("/foo/{bar}", (event) => {
  console.log("hi");
});

export const deleted = onRefDeleted("/foo/{bar}", (event) => {
  console.log("hi");
});

[inlined]

Some overall comments:

1. We're missing src/v2/database.js which is used to quiet the import linter (since the maintainer of the repo says we should pay them if we want a fix.
2. We've got an outstanding PR to fix some behavior around null handling in the RTDB. Did we incorporate any of that here? Can we incorporate it without doing a major version bump? We should figure out how to not throw away the author's work
3. In general I think _.isPlainObject is more strongly verifying things than we need. The only thing we need to check is whether something has typeof val === 'object' and that the type is not null.
4. We can ship the current path pattern work for now if we need to, but I strongly suspect we'll want to create a PathPattern utilty class like we have in One Platform code. Maybe PathPattern.compile(string) which can throw if there's a malformed path. Then something like hasParams() and matchParams(atual: string): Record<string, string>.

[egilmorez]

A couple nits for you Cole, thanks!

I'll take another look when only living code remains.

[colerogers]

@rhodgkins Yep that's the one. This will apply both to v1 & v2 functions.